High confinement, high power and high density operation is likely to face the H-mode density limit (HDL), making it a key issue for magnetically confined fusion. Despite its importance, the HDL is not nearly as thoroughly explored or understood as its counterpart in L-mode. Recent advances in the experimental characterization and physical understanding of the HDL are reviewed. There are several mechanisms limiting on the achievable density and for which theories have been developed in agreement with experiments. Contradictory observations have been collected in the different tokamaks so far. The good agreement between theory and experiment and the observed contradictions only allow the conclusion that in general competing mechanisms determine the achievable density in the H-mode.